The present invention is directed to monomers, oligomers and polymers comprising conjugated groups, methods for making thereof, and electroactive layers and devices comprising said polymers.
Electroactive devices are those in which the constituent materials' electronic properties change with respect to the environment and whose changes can be effectively used to convert one form of energy to another. Suitable examples of electroactive devices include, but are not limited to, photovoltaic devices, electroluminescent devices, electro-optical devices, and the like. Electroluminescent devices are structures that emit light when subjected to an applied electric field. In its simplest form, an electroluminescent device comprises a light-emissive layer between two electrodes. The cathode electrode injects negative charge carriers (electrons) and the anode electrode injects positive charge carriers (holes) into the light-emissive layer. Light emission occurs when the electrons and holes combine in the light-emissive layer to generate photons. As a practical aspect one of the electrodes is typically transparent, to allow the photons to escape the device. The light-emissive layer typically comprises a light-emissive material, often an organic material, which may be laid down as a film without substantially affecting the luminescent characteristics of the material and which is stable at the operational temperature of the device.
The color of the light generated by the light-emissive material is determined by the optical gap or bandgap of the organic light-emissive material, that is to say the difference in energy between the “highest occupied molecular orbital” (HOMO) and the “lowest unoccupied molecular orbital” (LUMO) levels. Effectively, the bandgap is the energy difference or energy gap between the valence band and conduction band. These energy levels can be estimated by photo-emission measurements and measurements of the electrochemical potentials for oxidation and reduction. The level of these energies is affected by numerous factors. Accordingly, the use of such energy values is indicative rather than quantitative.
Organic electroluminescent devices which use an organic material as the light-emissive material are known in the art. Among organic materials, simple aromatic molecules such as anthracene, perylene and coronene are known to show electroluminescence. U.S. Pat. No. 4,539,507 discloses the use of small molecule organic materials as the light-emissive material. Polymers are advantageous over small molecules when used in electroluminescent devices because polymer devices can be made on flexible substrates, and layers of the polymer may be put down by economical coating methods.
WO 90/13148 discloses an electroluminescent device comprising a light-emissive layer which is a polymer film comprising at least one conjugated polymer. In this case, the polymer film comprises a poly (para-phenylene vinylene) (PPV) film.
It is known to use a semiconductive conjugated polymer as the light-emissive layer in an electroluminescent device, for example from EP 0544795. EP 0686662 discloses a device for emitting green light. The anode is a layer of transparent indium-tin oxide. The cathode is a LiAl layer. Between the electrodes is a light-emissive layer of PPV. The device comprises also a hole transport layer of polyethylene dioxythiophene (PEDOT) which provides an intermediate energy level which aids the holes injected from the anode to reach the HOMO level in the PPV.
Polyphenylenes, polyfluorenes and other conjugated aromatic polymers are also well known as active layers in electroluminescent devices. Their synthesis and properties are given in, for example, U. Scherf et al., Adv. Mater., 14 (7), 477 (2002); and M. T. Bernius et al., Adv. Mater., 12 (23), 1737 (2000). These polymeric materials are generally prepared using aromatic coupling reactions, such as the Suzuki or Stille coupling, or the nickel catalyzed coupling reactions of aryl halides. Although the methodology for preparing these polymers has been well established, in many cases the coupling-polymerization reactions afford by-products that limit the molecular weight and may either quench fluorescence or induce significant red shifts in the emission spectrum of the polymer, thus limiting the color tunability of the corresponding electroluminescent devices. There is a need in the art to develop more versatile materials for applications in electroactive devices, which materials can be obtained economically in high yield.